Data transmission method and apparatus are described in EP-A-88 100 830 in which data from picture element (pixel) blocks are transformed into coefficients and afterwards coded as variable length words. The coefficients are recorded coded in blocks having a predetermined average length. Blocks which are not full are filled with intermediately stored data words from other blocks. In order to improve the immunity to interference, dc components and important alternating components of the transformed coefficients are recorded at fixed intervals. However, if faults occur in the transmission channel or in the storage medium, data of one block which has been distributed among other blocks may be difficult or impossible to decode.
For this reason, in accordance with PCT/EP91/01954, each of the blocks may additionally contain an address pointer which, upon the occurrence of transmission and/or read errors during subsequent decoding, points to addresses in memory from which data components separated from blocks having a greater than the average word length can be recovered and used for decoding.
Due to the additional address pointer information, for a constant channel capacity, there is a lower data rate available for picture data. This may lead to reduced picture quality during normal decoding.